The present invention relates to a method of excitation of a pressure sensor to increase the useful life of the sensor. This method of excitation is especially applicable to resistive sensors which are required to operate continuously for very long periods of time, and to sensors where low cost or operational requirements prevent the use of hermetic sealing or other means of protection from humidity and contaminants.
The present invention relates to electrical excitation of membrane-style pressure sensors, which are used in a variety of applications including automotive applications for horn activation or occupant weight sensing, and security applications. These sensors typically are constructed of layers of conductive ink and pressure sensitive material that changes resistance when a pressure is applied, printed on an electrical insulator such as a layer of plastic film. These sensors may be of the type as described in U.S. Pat. Nos. 5,398,962 and 5,563,354 to Kropp; and U.S. Pat. No. 5,541,570 to McDowell.
Normally in operation these sensors are supplied with a small DC voltage as an element in a resistive voltage divider or an operational amplifier circuit, and the resulting change of output voltage when force is applied is detected by an A/D converter, voltage comparator, or other detection circuit. The level of voltage sensed is proportional to the force applied.
Kropp describes in U.S. Pat. No. 5,398,962 a horn activator using pressure sensors. The Kropp sensor in effect is a variable resistor, which has an output that varies with the force applied. In Kropp, the resistance of the sensor is calculated using an inverting op-amp circuit. VO=VDxc3x97RF/RS (where RS is the sensor resistance; RF is the reference resistance; VO is the output voltage; VD is the driving voltage). However, Kropp describes a negative five direct current voltage being applied to the sensor.
Many of the applications for membrane sensors are subject to environmental conditions of high humidity and temperature extremes. In addition, many applications, such as automotive applications, require the sensor to continue to function for many years without failure while being continuously powered. It has been found that many of these membrane sensors have a limited life due to gradual degradation caused by ionic migration of conductive materials and electrolytic action inside the sensor when DC voltage is applied for a long period of time. This degradation may occur faster in the presence of humidity, which can gradually diffuse into the sensor through the thin substrate and through the adhesive layer which bonds the two substrates. The degradation of the sensor often causes a gradual reduction in the electrical resistance of the sensor. This degradation progresses until the sensor is electrically shorted or is no longer within its useful specifications.
It is desirable to minimize this degradation by providing an electrical excitation to the sensor that minimizes or eliminates the degradation of these sensors.
It is therefore the object of this invention to provide a method of excitation of a resistive sensor that maximizes the sensor""s life, allowing use of these inexpensive and flexible sensors for long service life applications. The excitation and sensing circuit must also allow the sensor to continue to perform its function of sensing pressure and be inexpensive so as to maintain the advantage of the low cost membrane sensor technology.
It was found by environmental testing that periodically reversing the excitation voltage polarity to a membrane pressure sensor can allow the sensor to continue to perform hundreds of times longer than an identical sensor which uses a constant direct voltage without polarity reversal.
Methods are shown which periodically reverse the sensor excitation polarity while still detecting a change of sensor resistance caused by application of pressure to the sensor. This method is referred to as Alternating Current (AC) Excitation.
The sensor applications shown are for an automotive horn system but this is simply one example of many uses for these sensors and the AC excitation methods shown here. Other resistive sensor applications include security applications, dental applications, and other automotive applications.
The preferred method of AC excitation described was developed to minimize cost and minimize the changes in hardware and software compared to the current DC excitation methods, while satisfying the sensing requirements.